Method of imaging transparent objects with coherent light



" U'IIIIE'U' ETHICS fare! iiiiiiiii p inventor Juris Upatnieks AnnArbor, Michigan Appl; No. 638,031 Filed May 12,1967 Patented Nov. 10,1970 Assignee Holotron Corporation Wilmington, Delaware a corporation ofDelaware METHOD OF IMAGING TRANSPARENT OBJECTS WITH COHERENT LIGHT 5Claims, 4 Drawing Figs.

U.S. Cl 350/3.5 Int. Cl G02b 5/02 Field of Search 350/35,

References Cited Armitage and Lohmann, Applied Optics. vol. 4, No. 4 pp.46! -467 (April 1965).

Gabor. Electronics and Power, pp. 230-234 (July 1966).

De Bitetto. Applied Physics Letters, vol. 8, No. 4, pp. 78-80 (February1966).

Suzuki, Japanese Journal of Applied Physics, vol. 5, No. 9, pp. 8 l4-8l7(September i966).

Primary Examiner- David Schonberg Assistant Examiner- Robert L. ShermanAttorney-Woodcock, Washbum. Kurtz and Mackiewicz phase modulator actingto scatter the light at very small angles.

METHOD OF IMAGING TRANSPARENT OBJECTS WITH COHERENT LIGHT BACKGROUNDOFTHE INVENTION 1. Field ofthe Invention This invention relatesgenerally to a method of producing images from transparent objects andmore particularly to such methods wherein a source of coherent light isused to illuminate a transparent objet to produce an image.

The invention of the la: or as a practical source of coherent light hasbrought about several applications wherein a transparent object isilluminated with coherent light to form an image. One of the principalapplications is in the field of microscopy where a thin section to beexamined is illuminated with light and viewed with an optical system.For some uses it is desirable to illuminate the object with laser lightbecause of its very intense brightness. Another related application isin the field of holography or microholography where it is desirable tomake a hologram of the specimen or object being illuminated. In thiscase it is necessary to use coherent light such as a laser to illuminatethe object. By producing a hologram, the wavefront coming from thespecimen is preserved and can be reconstructed at a later time for usewith one or several microscopy techniques.

2. Description ofthe Prior Art The use of coherent light to illuminatetransparent objects has given rise to problems not encountered inordinary incoherent imaging of transparencies. One major problemencountered is the formation of spurious diffraction patterns, usuallyappearing as circular Fresnel zone patterns, resulting from such thingsas dust or dirt on the transparent object or imperfections in an opticallens system used in the imaging process. These dust particles or lensimperfections diffract the light so as to cause interference between thediffracted light and undiffracted light, thereby creating theundesirable interference ring pattern at the image plane. Theseundesired ring patterns are analogous to noise in a communication systemand can be eliminated with the utilization of a diffusion plateinterposed between the source of coherent light and the transparentobject being imaged. The diffusion plate, in effect, destroys theregularity of the spatial amplitude and phase of the illuminatingwavefront without affecting the temporal coherence thereof. That is, theobject transparency is illuminated with a wavefront having a phase thatvaries randomly from point to point thereacross in a time invariantmanner. This method effectively spreads out the noise over the entireimage plane, thereby rendering it virtually invisible to the observer.The use of a diffusing plate in a coherent light transparency imagingsystem is more fully explained in the November I964 edition of theJournal of the Optical Society of America, beginning at page I295 andentitled Wavefront Reconstruction With Diffused Illumination and3-Dimensional Objects by Emmet N. Leith and Juris Upatneiks.

While the utilization of a diffusion plate in an imaging system of thetype described effectively eliminates the problem of spuriousdiffraction patterns or noise, it creates another problem which in someapplications is very objectionable. This problem now referred to is theproduction of a very discernible grainy background to the imageresulting from the utilization of a diffusion plate in the illuminatingbeam. This grainy background is believed to be a result of the randomamplitude produced by the diffusion screen. The grainy pattern, whilenot particularly objectionable in imaging objects of ordinary size,becomes a real problem in microimaging systems wherein the order ofmagnitude of size of the objects being imaged are of the same order asthe grains themselves.

It is therefore an object of this invention to provide a method ofimaging transparent objects with coherent radiation which eliminatesboth the noise and the grain heretofore present.

Another object of this invention is to provide a method of producingholograms of transparent objects which are capable l of reconstructingimages thereof which are relatively free of undesirable noise and grain.

SUMMARY OF THE INVENTION Briefly, the objects of this invention areaccomplished in a method of imaging transparent objects with coherentlight wherein a phase modulator is placed in the system between thesource of coherent light and the transparent object being imaged, thephase modulator having the property of randomly scattering theilluminating light at small angles thereby imparting a random phase onthe illuminating beam while the amplitude thereof remains constant.

While the process which is regarded as the invention is distinctlypointed out and described in the appended claims, the underlyingprinciples of this invention, together with a detailed descriptionthereof, may be more readily apparent with reference to the followingdescription taken in conjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. IA illustrates an embodiment ofthis invention in which a hologram of a transparent object is produced;

FIG. lBillustrates a reconstruction process from the hologram made inFIG. IA;

FIG. 2 illustrates a preferred arrangement of a portion of the systemillustrated in FIG. 1A; and

FIG. 3 illustrates schematically a specific application in which thepresent invention may be employed.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. 1A, the inventionis illustrated in an embodiment including an illuminating source ofcoherent radiation 11, which may be a laser, which produces a beam ofspatial and temporal coherent radiation 13, a portion of which isdirected to pass through a nondiffuse transparent object 15. Anondiffuse transparent object is any transparent object that does notscatter light at wide angles. Almost all transparent objects arenondiffuse; however, in the case of a diffuse transparent object, suchas a ground glass diffusion screen, the present invention is notapplicable, inasmuch as such an object practically destroys theamplitude and phase regularity of any light passing therethrough. Theobject 15 is preferably a two-dimensional object. However, the inventionis applicable in the case of a three-dimensional transparent object ofwhich only a selected planar portion is desired to be imaged.

The object I5, after being illuminated by beam 13 may be imaged by meansof an optical lens system (not shown) such as in ordinary microscopy, ormay be utilized to produce a hologram of the object 15 as is illustratedin FIG. IA. In FIG. IA, the beam 13 impinges upon a hologram detectorsurface 17 positioned at the hologram plane and is caused to interferewith a reference beam I9 derived from the same source 11 so as to becoherent therewith. The method for producing the reference beam I9 mayinclude a beam splitter 21 inserted in the path of the beam l3, a mirror23 to deflect the beam 19 to the hologram detector surface 17, and alens 25 to make the reference beam diverging if so desired. The methodof producing a hologram as thus far described in connection with FIG. 1Ais now well known to those skilled in the art and no detaileddescription of this method is deemed necessary for the present purposes.Such a detailed description may be found in the US. application for Pat.Ser. No. 503,993, filed Oct. 23, I965.

Reconstruction of an image of the object 15 from the hologram producedin FIG. 1A is illustrated in FIG. 1B in which .a beam 19a is directed tothe hologram 17' at the same angle as the reference beam 19 took duringthe construction process. An image 15a will appear in an image planespaced from the hologram at a distance determined by the relativecurvatures between the reference beam 19 in the construction process andthe illuminating beam 19a.

If, during the construction of the hologram, there existed any dust orother imperfections in the system, the reconstructed image a willinclude diffraction patterns which are objectionable because they areregular in nature and thus draw the attention of the observer. Also, ifa transparent-type object is being imaged with coherent radiation,imperfections in the imaging system utilized will cause such diffractionpatterns at the image plane.

By introducing a phase modulator 27 directly in front of the transparentobject 15 during the construction process of FIG. IA, the resultinginterference patterns in the reconstructed image lSa due toimperfections or dust in the system become irregular and thus relativelyinvisible, due to the random phase of the illuminating light. Byimposing the requirement on the phase modulator 27 that the amplitude ofthe beam l3 passing therethrough remain constant, the image producedwill be free of the grainy structure that is characteristic of imagesproduced with diffused coherent light.

In order to prove that the desired results are truly accomplishedaccording to the teachings of this invention, a mathematical analysishas been performed with respect to both the elimination of regulardiffraction patterns and the elimination of the grain pattern backgroundat the image plane. Without going into this mathematical analysis (forit is quite extensive and complex and is not necessary in the basicunderstanding of this invention) it suffices to say that in analyzingthe mathematical expression for the wavefront at the image plane andconsidering the desired results intended to be accomplished, it wasshown that in order to reduce the noise and the grain effects in theimage, the phase term and the amplitude term of the illuminating lightbeam 13 were required to be respectively random and constant.

The phase modulator 27 may be of any desired type so long as thecharacteristics of random phase and constant amplitude are met. Incarrying out actual tests of this invention, it has been found that sucha phase modulator in its preferred form comprises a phase plate thatscatters the illuminating light over very small angles in a randomfashion. Such a phase plate may comprise a sheet of ordinary antiglareglass, one side of which has been polished flat. Another embodiment ofsuch a phase plate comprises a sheet of glass or other transparentmaterial in which shallow etches have been provided, or, alternatively,on which a special coating of suitable material has been applied.Additionally, in some cases it may be desirable to direct the objectbeam to a reflective phase modulator (which may be a mirror with anirregular surface) and then produce an image of the reflective phasemodulator at a position coincident with or close to the object. Itshould be understood that the particular phase modulators disclosedherein are presently considered preferred forms of the phase modulator27, but in no way limits the invention to such forms.

It has been found that the best results are obtained when the phasemodulator 27 is positioned as close as possible to the transparentobject IS. The reason for this is that when the phase modulator isspaced away from the object, interference may result due to the angularscattering of the light, result of which would be a modulation of theamplitude of the light beam 13 passing through the object 15. Thespacing distance must be small enough so that the light scattered by thephase modulator impinges on the transparent object before it hastraveled far enough to interfere with light from an adjacent osition onthe phase modulator.

Referring to FIG. 2, in a preferred form of the present invention animaging system comprising a pair of lenses 29 and 31 are utilized toproduce an image 27a of the phase modulator 27 at an image plane whichis coplanar with the plane of the object 15. Although the imaging systemof FIG. 2 is preferred, it is possible that with good quality phasemodulators in which the angle of modulation is very small, a certainallowable separation distance exists between the phase modulator and theobject and therefore it is not intended that the invention be limited tothe embodiment illustratedin FIG. 2.

In the detailed mathematical analysis carried out in conjunction withtesting the basic principles of this invention, it was determined thatthe lack of grainy structure at the image plane due to the employment ofthe phase modulator 27 occurs only where the amplitude of the imagecarrying wavefront is constant and further, that this constant amplitudewavefront occurs only at the image plane of the imaging system utilized.if any. Furthermore, it was determined that the frequency spectrum ofthe image carrying wavefront must be within the band pass of the opticalimaging system utilized, or else the amplitude at the focal plane willnot remain constant and the grainy appearance will be present. Thisobservation puts the further requirement that the aperture of theimaging system be large enough to include the entire frequency band ofthe illuminating wavefront.

These observations give rise to specific applications of the discoveriesunderlying this invention, one of which is illustrated in FIG. 3. InFIG. 3 similar elements as those in FIG. IA areutilized and are givenlike reference numerals. However, in FIG. 3 a system for imaging atransparent object is depicted rather than the system for producing ahologram as illustrated in FIG. 1A. Therefore, FIG. 3 includes a pair oflenses 33 and 35 utilized to produce an image of the object 15 at animage plane 37 In FIG. 3, two additional planes 37a and 37b areillustrated on either side of the focal plane 37 of the imaging system.According to this invention, the grain free image will appear at thefocal plane 37, but an out of focus grainy image will appear at both theout of focus planes 37a and 37b. By moving the image plane 37 along thepath of the illuminating beam 13, for example. between the limitsillustrated by planes 37a and 37b, the focal plane of the imaging systemcan be determined simply by observing at what position the grainyappearance disappears. It will be apparent that this method can also beused to determine any curvature existing in the field of the imagingsystem.

Another application of this invention utilizes a set of calibrated phaseplates to determine the frequency response of an optical imaging system.In order to eliminate the grainy appearance at the image plane 37, thefrequency band pass of the optical system used for imaging must be aslarge as the frequency bandwidth of the phase plate. That is, theproperties of each phase plate can be represented by random phasechanges over a frequency range, and in order to eliminate the grainyappearance, the imaging system must be able to transmit signals overthis frequency range. If the band pass of the imaging system is lessthan the bandwidth of the phase plate, then the grainy appearance willnot be eliminated. Thus, with a set of calibrated phase plates, each ofa known bandwidth, the frequency response of an imaging system can bedetermined simply by observing which of the calibrated phase plateseliminates the grainy appearance at the image plane 37.

While the invention has been described in somewhat general terms withoutreference to a specific detailed and complex mathematical analysis, andwhile the invention has been illustrated in specific embodiments, it isnot intended that the invention be limited to the specific embodimentsillustrated. It is also considered desirable that the mathematicalanalysis be eliminated, inasmuch as it adds nothing to the understandingof the basic principles underlying this invention and may tend toconfuse those not intimately familiar with mathematics on the levelrequired by such analysis.

lclaim:

I. In the method of illuminating a nondiffuse transparent object bypassing therethrough a coherent light beam having a time invariant phasefunction thereacross, the improvement wherein a surface of said objecttransparency is illuminated with a surface of said coherent light beamhaving a uniform amplitude and randomly varying phase thereacross,whereby an object bearing coherent.light beam is produced that containsinformation of said object transparency surface with reduced noise.

2. In a method of illuminating a nondiffuse transparent object bypassing a coherent light beam therethrough, the improvement comprisingthe step of modifying said beam to have a uniform amplitude and timeinvariant randomly varying phase across said object by placing a phasemodulator in said beam before striking the object and in close proximityor coincident with said object. whereby an object bearing coherent lightbeam is produced containing information of said object with reducednoise.

3. In a method of illuminating a nondiffuse transparent object bypassing a coherent light beam therethrough. the improvement comprisingthe step of modifying said beam to have a uniform amplitude and timeinvariant randomly varying phase thereacross in a plane of said objectby placing a phase modulator in said beam before striking the object andimaging said modulator to a position in space coincident with saidobject plane. whereby an object bearing coherent light beam is producedcontaining information of said object plane with reduced noise.

4. The method as defined in claim 2 wherein said phase modulator ischaracterized in that it scatters illuminating light at small but randomangles.

5. The method as defined in claim 4 wherein said phase modulatorcomprises a sheet of nonglare glass, one side of which has been polishedflat.

